Optical components of the above kinds are in general made by moulding in a plastics material, such as a thermoplastic, for example polymethylmetacrylate (PMMA) in a mould. The mould may consist of several parts, this being determined by the complexity of the form of the optical component to be made.
Reference is here made to FIG. 1 of the accompanying drawings, showing part of a known type of mould for making optical components with discontinuous surfaces. Such a mould includes laminar inserts, such as those indicated in FIG. 1 at 10, 20, 30 and 40. These consist of thin plates or shims which are machined to a very high degree of accuracy, this accuracy being also required by the form of the component to be moulded. It is determined by the surfaces 11, 21, 31 of the respective inserts, which form in the mould cavity itself the working surfaces of the optical component. A similar degree of accuracy is also required by the faces 12, 13, 22, 23, 32, 33, 42 and 43 of the inserts which are arranged to come into mutual contact and to form neutral surfaces of the optical component. Each of these thin plates corresponds to one of the discontinuities in the optical component to be moulded.
By convention, in the present description, a surface which plays any part in the forming of a light beam, and through which light rays emitted by a light source pass, will be called a working surface. Similarly, a surface which has no effect on the formation of a light beam, and through which no light ray passes, will be referred to as a neutral surface.
The insert plates are machined, and are then assembled together and held together so as to form the walls of the required mould, complementary to the surfaces of the optical component to be made. Their assembly also calls for high precision, if it is required that the moulded component shall have the required optical qualities. As a result, a mould of this kind is relatively difficult to make, because of the high accuracy required in making the insert plates and the accuracy with which these plates must be adjusted among themselves, and also because of the requirements for generally holding the mould together.